Uniaxial microphone



June 8, 1954 H. F. oLsoN ET AL- UNIAxIAL MICROPHONE Filed Nov. 30,'19512 Sheets-Sheet l s l ISIS l s Y R ,N @www E N .L E E EDEL v .EB mFPE.YNN mhh HDD Hdd ATTRNEY June 8, 1954 H. F. OLSON x-:T AL

UNIAXIAL MICROPHONE 2 Sheets-Sheet 2 Filed Nov. 30, 1951 200 460 idd IM@unz 0 E+. H Y T 1l E E M NnUE l .R WERE. m N PE M mmm Y Hdd B PatentedJune 8, 1954 UNAXIAL MICROPHONE Harry F. (ilson, Princeton, JohnPreston, Metedeconk, and John C. Elea-soy, Mercerville, N. J., assignorsto Radio Corporation of America, a corporation of Delaware ApplicationNovember 30, 1951, Serial No. 259,138

ll Claims. (Cl. 179-1155) The present invention relates to microphones,and more particularly to ribbon type unidirectional microphones.

W ith the advent oi television, efforts have been Vmade to makemicrophones smaller, less obtrusive, without reducing the quality of theperformance of the unit to a degree substantially below that of thelarger, more conventional In the development of a small microphone asset forth, a motor mechanism was chosen which is substantially similarto that employed in the microphone disclosed in the aforesaid patent,2,566,094. However, the microphone of the present invention does notinclude a sound transmitting tube on the sound input side of the ribbonas does the pressure responsive micromcrophones. In the class ofmicrophones 13110116. Consequently u relatively lelge flat Sul"- Whereinthe sound source is relatively close, such l fue@ iS presented t0 thempngirlg Sound Waves as hand-held microphones, considerable adin theVicinity of the ribbon. Such a iinite flat vancernent has been made. Amicrophone of SuIeCe, broken by en air gap in which the ribbon that typeis shown in Olson et al, Patent Num.. is supported, produces adiffraction interference ber 2,566,094. That microphone is characterinthe risher frequency sound waves. The inized, however, in that it is apressure responsive l5 terelele, thus produced, dStOltS the TeSlOOllSeunit and, hence, has a non-directional response 0f the mClOphOne t0 theSOuIld Waves implsns characteristic. on the ribbon.

For sound pick-up froma more remoto position, It is, therefore, afurther object of the present as from a boom, wherein the microphone isat invention to provide a microphone as Set forth a considerabledistance from the sound source, 2O Whelel means are provided t0 preventthe deit has been found desirable to use a microphone velopment ofdiiraction interference in the sound having a unidirectionalcharacteristic. The Waves imlonsng on the ribbon. microphones which haveheretofore been used for ln accomplishing these and other objects, thereboom operation have been rather large because has been provided, inaccordance with the present of the structure thought necessary toproduce invention, an improved pressure gradient I'eSpOIla pressuregradient responsive type microphone Sve, ribbon type microphone- ThemCrODhOIle having the desired characteristics. is characterized in thatthe motor element is Since the hand-held type microphones arepriarranged With the ribbon supported in an air marily used to pick up aspeaking or singing gap in One end 0f e megnet Structure. Thus, Voice,the structure may be quite simple. Howa relatively Wide flat surfacewould be presented, ever, the remote or boom microphone may be in thevicinity of the ribbon, te the impngng used to pick up dramaticproductions, including sound waves. Anti-diffraction lobes are securednumerous sound effects. Among the various to the fiat surface of the endof the magnetic sound effects, many are accompanied by the genstructureon each side of the air gap to prevent eration of an initialhigh-amplitude, low frethe diiiraction of the sound Waves by thesurquency pressure wave. Such a pressure wave faces adjacent to the airgap. In addition, a accompanies a blast from a firearm. If the pluralityof baie screens are interposed between microphone is relatively close tothe source of the sound source and the ribbon to absorb or the pressureWave, the ribbon of the microphone dissipate an initial high amplitudelow frequency may be permanently damaged. pressure Wave, such as wouldaccompany the r- It is accordingly an object of the present ining of a.45 caliber pistol, to protect the ribbon vention to provide an improvedpressure gradient against damage by such a pressure wave. responsiveribbon typo microphone o horootorizod A better understanding of thepresent invention in that a smaller sized unit 1s obtained Without maybe had from the following detailed descrip.. Sacmcmg mamy 9iPerformance' 45 tion when read in connection with the accom- It 1s.another oblect of the present invention panymg drawings in whichbggvg;earsgleldrgsln Figure l is an elevational view partly incrossvided for protecting the ribbon from damage by .sect-'1011? fn? amlcrophone embodymg the present the initial pressure Wave such as wouldaccom- ,o mvn lon pany the firing orare sauber pistolet close range.Fltiufe 2 1S a V1eW tak en 310D? th? 1111 2-2 Still another object ofthe present invention of Flgure 1 and lookmg m the duecton 0f the is toprovide a microphone as set forth wherein appended arrows means areprovided for dissipating the energy Figures 3 and 4 are VeWS ShOWIlg thelabyrinth of the initial pressure Wave before it impinges connector,

on the ribbon.

Figures 5 and 6 are graphs indicating the frequency responsecliaacteristic, respectively, with and without the use of theanti-diffraction lobes,

Figure 7 is a fragmentary view partly in section similar to Figure 1 butincorporating a modication of the invention, and,

Figure 8 is a diagram of an electrical circuit analogous to theacoustical circuit of the microphone of the present invention.

Referring now to the drawings in more particularity, there is shown inFigures 1, 2 and? an outer casing 2 within which a microphone is housed.The microphone motor assembly includes a pair of magnetic members Il and6 which are secured, at one end, to a magnetically permeable base member3. A pair of soft iron pole pieces It and I2 are secured to the magneticmembers at the end opposite from the base member 8. The.mag.- neticmembers are approximately semi-cylindrical in shape and are mounted uponthe base meinber with their flat surfaces facing each other 1n spacedapart relation. Similarly the pole pieces ID and I2 are fiat andapproximately semi-cylindrical. These pole pieces are secured to one endof the magnetic members respectively and are also arranged with theirstraight side in spaced apart relation facing each other. Thus, the polepieces denne an air gap. A conductive vibratile member or ribbon ld issupported for vibratory movement in the air gap. A motor unit of thistype is selected because it presents several advantages, among thembeing simplicity and magnetic eiiiciency. Simplicity is accomplished bythe use of a small number of easily machined parts. High magneticefficiency is obtained due to the small flux leakage inherent inmagnetic designs of this t e.

ydjacent to the end of the motor assembly opposite from the air gap,there is an acoustical labyrinth iii substantially as defined in OlsonPatent Number 2,271,988. In the space between the magnetic members l and5 there is positioned a conduit or connector I6 which connects one sideof the air gap through an aperture I8 in the base member 8 to theacoustical labyrinth I5. The construction of the connector IB issubstantially the same as that described in the aforesaid Patent2,566,094, with the exception that there is provided one or moreorifices I'I (two being shown) near the end of the conductor adjacent tothe air gap as shown in Figures 3 and 4.

Bridging the space between the magnetic members along the peripheraledges there is positioned a perforated metal screen 2D. A layer ofclosely woven fabric such as silk 22 is secured to the back of the metalscreen. A similar piece of fabric covers the orifices Il in theconnector I6. The space between the connector and the metal screens isfilled with an acoustical damping material such as felt or iiber glass24.

Secured to each of the pole pieces IB and I2 on the flat surfacesthereof opposite from the inagnetic members, there is ananti-diffraction lobe 26. These lobes may be made from any suitablematerial. However, a non-magnetic material is preferred. Incross-section, the lobes are substantially semi-circular in shape andare positioned on the pole pieces closely adjacent to the air gap.Between Vthese lobes there is positioned a plurality of successivelyspaced baffles 28-3 which includes a perforated metal support with abacking of a closely woven fabric. The spacing of the baffles is suchthat sounds impinging on the ribbon I4 must first pass'through one ofthe bafies and then the other baffle before arriving at the ribbon. Theend closure of the outer` casing is formed of a perforated metal supportscreen 32 and having a fabric backing 34. However, here the dimensionsof the perforations and the fabric of the backing are such that they aresubstantially transparent to the sound waves.

In Figures 1 and 2 the bafes 28-30 bridge the space between theanti-diffraction lobes. The perforated metal screen constituting the endclosure of the casing has portions 36 which extend longitudinally of thecasing. These extended portions are arranged to be aligned with theorifices I'I in the connector I6. The fabric backing also extends alongthe extended portions. The perforated metal screens 20 are substantiallyparallel to the extended portions of the end closure and are spacedtherefrom. In Figure 7 the construction of the baffle is somewhatmodified in that the bafes are in the form of boxes 28a, 33a, one withinthe other, which are substantially pyramidal in shape. The inner box 28ais spaced from the outer box 30a as are the bafiles 28-30 of Figures 1and 2.

A microphone thus constituted may be termed a uniaxial microphonebecause the axis of maximum sensitivity corresponds to the axis of theassembly. The orifices I'I in the labyrinth connector I6 permitvariations in sound pressure to impinge on the back side of the ribbonI4. However, the structure of the microphone is such that the soundwaves impinging on the back side of the ribbon are delayed and shiftedin phase with respect to the sound waves falling on front of the ribbon.So long as the sound source is disposed along the axis of themicrophone. on the front side of the ribbon, a maximum phase differencebetween the sound Waves falling on the front and back of the ribbonoccurs, resulting in a maximum output. However, deviations from the axisare accompanied by a corresponding reduction in the phase difference anda lower output, with maximum cancellation occurring when the soundsource is disposed along the axis of the microphone and on the back sideof the ribbon. If the microphone as described were to be employedwithout the use of the anti-diffraction lobes, the flat surfaces of thepole pieces would produce a diffraction in the sound waves which wouldin turn cause an irregularity in the response characteristic of themicrophone substantially as shown in the chart of Figure 5. There it maybe seen that as, the frequency of the incident sound waves increases, anirregularity in the response of the microphone is produced in the upperportion of the audible frequency range. It has been determined that thisirregularity is a result of the diffraction of the sound waves as theyimpinge on the flat surfacesr of the pole pieces. When theanti-diffraction lobes are positioned on the pole pieces, the responsecharacteristic is substantially as shown in Figure 6. There it may beseen the irregularities have been smoothed out and substantiallyeliminated. In addition, the contour of the lobes is such that theyeffectively constitute a small horn, thereby increasing the sensitivityof the microphone.

When the microphone is used to pick up such sounds as the discharge offirearms, the ribbon may be permanently injured by the initiallowfrequency, high amplitude pressure wave which accompanies the blast ofsuch rearms. This initial pressure wave, of course, varies in timeduration with the type of instrument producing the wave. Forv example,the time might var-y from a 20th of a second for a .454 caliber pistolto a liOth of a second for a .22 caliber pistol. These Ydispensed with,without low frequency pressure waves are not reproduced through thecomplex chain of elements which constitute the sound channel ofrecording and broadcasting equipment. However, it is the low frequencycomponent in the blast pulse that produces the large deflection in theribbon and -stresses it beyond the elastic limit and thereby introducesa permanent deformation in the ribbon. Since the initial pressure waveforms no part of the reproduced sound it may readily be materiallyaffecting the desired sound effect. In accordance with the presentinvention this initial pressure wave is dissipated before it impingesupon the vibratory member. The system of successively spaced bafesprovides means for so dissipating the energy of the blast. Consider sucha sound wave as emanating from a point beyond the perforated end closureof the microphone. The pressure wave will first encounter baiTle 3] or30a where it is attenuated by the acoustical resistance and theinertance thereof. From thence it falls upon the inner baie 23 or 28awhere, again, it is attenuated. By this time the pressure wave has beenattenuated to the extent that it will no longer have a deleteriouseffect upon the ribbon. Similarly the sound wave impinges upon theopposite side of the ribbon after having passed the perforated metalscreen 20, the acoustical damping material 24, and the orifices Il, thusattenuating the energy of the pressure wave impinging upon the reverseside of the ribbon. Although the acoustical impedance of the bafflestructure is substantially constant for all frequencies, effectively, itconstitutes a low frequency acoustical impedance. This is true becausethe impedance of the baie structure must be compared with the acousticalimpedance of the ribbon itself. For low frequencies, the acousticalimpedance of the ribbon is very small in comparison with the impedanceof the baffles. Hence, for low frequencies the dominant impedance isthat of the baffles. On the other hand, for higher frequencies, theacoustical impedance of the ribbon dominates that of the baffles.

The acoustical network shown in Figure 8 is representative of theconditions prevailing in the microphone herein set forth and may provehelpful to a better understanding of the principles involved herein.

In that network,

P1 is the sound pressure on the front of the microphone,

MA1 and Rm are the inertance and acoustical resistance respectively ofthe air load on the front of the microphone,

MB1 and Rm are the inertance and acoustical resistance respectively ofthe first baille 30, while MBi and Rsl are inertance and acousticalresistance respectively of the inner baffle 28,

C1 is the acoustical capacitance of the volume between the baflies 28and 30 or 28a and 30a, C'1 is the acoustical capacitance of the volumebetween the inner baffle and the ribbon,

Ms and Rs are inertance and acoustical resistance of the slit betweenthe edge of the ribbon I4 and the pole pieces I0 and l2,

MR, RR and CR are respectively inertance, acoustical resistance andacoustical capacitance of the ribbon,

ZE is the acoustical impedance due to the electrical circuit associatedwith the ribbon,

P2 is the sound pressure on the back side of the vibrating system,

MAZ and RA2 are respectively the inertance .and acoustical resistance ofthe air load of the back side of the vibrating system,

M132 and Rs2 are respectively inertance and acoustical resistance of thebaffles 20 on the side of the microphone,

C2 is the acoustical capacitance of the volume behind the baffles 20,

M2 and R2 are the inertance and acoustical resistance respectively ofthe screen covering the orifices I1 in the labyrinth connector I 8,

M3 and R3 are the inertance of acoustical resistance respectively of theorifices in the labyrinth connector,

RP is the acoustical resistance of the labyrinth In the event that theacoustical damping material is used in the space between the baies 20and the labyrinth connector I6, the acoustical network would be modifiedto the extent of having the acoustical resistance of the dampingmaterial connected in shunt across the acoustical capacitance Ccz.

Similarly, acoustical damping material may be inserted between the frontbaiiies 28 and 30 or 28a and Sila. This would, in effect, modify theacoustical network by the inclusion of the acoustical resistance of thematerial connected in shunt with the acoustical capacitance C1 of thevolume between the bailles.

Thus it may be seen that, by controlling the values of the elements ofthe network, through the control of the dimensions of the componentsrepresented thereby, the very low frequency pressure waves may beeffectively filtered out, without materially affecting the microphonesresponse to the desired sound waves.

From the foregoing, it may be seen that there has been provided animproved small-size pressure gradient responsive microphone wherein theribbon is protected from damage by high amplitude, low frequencypressure waves, and wherein means are provided for preventing theformation of diffraction patterns which would distort the microphonesfrequency response characteristic.

What is claimed is:

1. A pressure gradient responsive ribbon type microphone comprising amagnetic field structure generally cylindrical in shape havingsubstantially flat circular ends, said field structure defining anelongated air gap in one of said circular ends, a conductive vibratilemember supported in and substantially coextensive with said air gap, aplurality of successively spaced bales effectively constituting a lowfrequency acoustical impedance interposed between said vibratile memberand the ambient, and a pair of antidiffraction lobes mounted on said endadjacent to said air gap..

2. The invention as set forth in claim 1 wherein said anti-diifractionlobes are substantially semicircular in cross section.

3. The invention as set forth in claim 2 wherein said lobes are securedto said end on opposite sides of said air gap.

4. The invention as set forth in claim 3 characterized in that at leasttwo of said baffles are disposed between said lobes bridging the spacetherebetween.

5. The invention as set forth in claim 1 wherein said bailies compriseperforated metal supports and a layer of closely woven fabric secured toone surface of each of said supports.

G. A pressure gradient responsive microphone comprising a magnetic eldstructure having an air gap in one end thereof, a conductive vibratilemember supported in and substantially coexn tensive with said air gap,said magnetic structure being disposed substantially all on one side ofsaid air gap, a plurality of successively spaced bafiies effectivelyconstituting a low frequency acoustical impedance interposed betweensaid vibratile member and the ambient, and a pair of anti-diifractionlobes secured to said structure adjacent to said air gap.

7. A pressure gradient responsive microphone comprising a magnetic iieldstructure having an air gap in one end thereof, a conductive vibratilemember supported in said air gap, said magnetic structure being disposedsubstantially all on one side of said air gap, and a pair ofanti-diffraction lobes secured to said structure adjacent to said airgap.

8. The invention as set forth in claim "I wherein said magneticstructure is generally cylindrical in shape having substantially flatcircular ends, one of said ends being divided thereby defining said airgap.

9. The invention as set forth in claim 8 wherein one of said lobes issecured to each portion of said divided end.

10. The invention as set forth in claim 9 wherein said lobes aresubstantially semi-circular in cross section.

11. A pressure gradient responsive ribbon type microphone comprising amagnetic field structure generally cylindrical in shape and including aspaced pair of substantially semi-cylindrical magnetic members and apair of fiat semi-cylindrical pole pieces positioned on one end of saidmagnetic members, said pole pieces defining an elongated air gap, aconductive vibratile member supported in said air gap, an acousticlabyrinth adjacent the end of said magnetic structure opposite from saidair gap, a connector member of substantially constant cross sectionalarea connected between said air gap and said labyrinth, said connectormember having a sound admitting aperture adjacent to the end thereofnearest said air gap, a plurality of successively spaced bafeseffectively constituting a low frequency acoustical impedance interposedbetween said vibratile member and the ambient, and a pair ofanti-diffraction lobes respectively mounted on said pole pieces.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,348,356 Olson May 9, 1944 2,566,094 Olson et al Aug. 28,1951

